A typical cellular radio communications system comprises a number of (radio) base stations (RBSs) providing communications resources to mobile units and user equipment present in the system. A base station often has multiple (e.g. at least two) associated directional antenna units that are able to provide the communications services through several separated sectors or cells within the total coverage area of the base station.
In order to enable seamless movement of a mobile unit between different cells during a communications session, the radio coverage areas of neighboring cells typically, at least partly, overlap. Such an overlapping coverage area is denoted handover area or region in the art.
The size of the handover area depends on measurement control parameters. A minimum region requirement is that a mobile unit travelling from one base station and/or cell to another has time to measure, report, configure and synchronize to the new base station and/or cell before the communications link to the old cell has to be dropped due to insufficient signal quality.
There is a clear distinction in operation between moving from cells on different base stations (or sites) and cells on the same site, the former being denoted soft handover and the latter is a so-called softer handover.
For softer (intra-site) handover, the single base station simultaneously transmit, using its different directional antennas, the same information over each of the cells to the mobile unit, thus, creating transmit macro diversity gain. Correspondingly, signals communicated from the mobile unit are received through the different cell radio hardware (antenna equipment) in the single base station. The received unprocessed signals from these cells are then usually directly combined using a maximal-ratio combining (MRC) or equivalent configured receiver or detector. MRC is generally superior compared to the information combining techniques used for soft handover, where the mobile-unit-originating data are received and detected by different base stations.
However, softer handover and MRC signal reception implies restrictions and complexity to the antenna-related equipment in the base stations. For example, in softer handover all internal cells radio chains of a given base station have to be fully accessible for all receivers in the base station. This complexity, thus, will be affected by the number of diversity paths or radio chains available, which can be quite high, especially for multi-antenna based RBS configuration or arrangement. In addition, the increased number of communication paths for the signal, due to the softer handover with corresponding control signaling in the radio access network, results in more signals transmitted by the base station that, by constructive combining of the transmitted energy, in turn leads to an increased coverage and/or transmission area. This increased coverage and/or transmission area will effect the interference situation in the system by increasing the interference levels. The increased number of communications paths also takes capacity resources from the system. In summary, seamless mobile communications utilizing softer handover and MRC generally yield the best statistical reduction of fading, however, this comes as a cost of more complex antenna equipment, increased interference and reduced capacity.